Today's fiber optic based networks use transceivers as the interface between electronics and optical signals that propagate on the optical fiber and at other points in the network where information is converted between electronic form and optical form. Two critical subsystems of a transceiver are the optical transmitter and the optical receiver. The wavelength of the light emitted from the optical transmitter, and in certain cases used in the optical receiver, is an important parameter used towards designing, constructing and operating fiber optic links, transmission systems and networks. Today's fiber optic transmitters and some coherent optical receivers predominately use lasers that emit light at a fixed wavelength. Employing lasers with the capability to tune the emission optical wavelength addresses under the control of an electronic control signal or set of signals solves many issues that exist today with links and networks that use fixed wavelength transmitters and receivers. Additionally, widely tunable wavelength semiconductor lasers that can be tuned over large wavelength ranges are a key component for today's and future optical communications systems and networks to reduce cost of designing, building, operating and maintaining and increasing the flexibility of such links and networks. There are many advantages to using a widely tunable laser over fixed wavelength laser, namely, one laser can be used as a single part number for a build of materials to construct a network that uses many wavelengths or to replace one of many different wavelength lasers in the field instead of requiring a spare laser for each wavelength to be kept in stock. Widely tunable typically refers to a large tuning range Δλ relative to the wavelength of operation λ, such that Δλ/λ is as large as possible. For example in a λ=1550 nm communication system, a Δλ tuning of approximately 20 nm to over 100 nm would be considered widely tunable for today's applications. Tunable lasers also allow more flexibility in designing the transmission or optical network system potentially lowering costs in the planning and build-out stages of network deployment. In order to make tunable lasers and tunable optical transmitters cost, power and size or density efficient, monolithic integration of the tunable laser and a subset or all of the associated transmitter or receiver elements using a monolithic substrate like a semiconductor, is required and also leads to improved performance and reliability, all key factors in today's fiber communications systems and networks.
An integrated widely tunable laser typically consists of multiple sections, generally including a gain section, a tunable phase section, and tunable mirror sections and in some designs a tunable filter section is also incorporated. Tuning the physical parameters of these sections is achieved using electrical control signals, thermal tuning or some other mechanism that changes the refractive index or other property of the laser elements, and results in tuning of the output laser wavelength. It is desirable to integrate additional elements with the laser to perform key functions associated with wavelength tuning, for example, power monitors and optical amplifiers to boost signal power and maintain constant power with optical power gain control loops as the wavelength is tuned as well as elements to control elements to lock the tunable wavelength (or frequency) to a desired stability and accuracy or preset standardized frequency grid. One class of tunable lasers utilizes mirrors with periodic reflection peaks (periodic in wavelength or optical frequency) where the two mirrors reflection periods are not spaced with the same periodicity, and tuning is achieved when one of the peaks of each mirror overlap known as the Vernier effect and the laser optical emission occurs predominately at the wavelength where the mirror reflection peaks overlap. It is also necessary that the optical gain inside the laser cavity is properly aligned with the location of the mirror reflectivity peaks and the overlapping tuning wavelength in order to ensure emission at the desired wavelength. Techniques to measure, characterize, monitor and control the gain and mirrors are critical in realizing practical tunable lasers and transmitters that can be manufactured at low cost. The Vernier effect has provided excellent performance characteristics in terms of the quality of the output optical wavelength and can be achieved with a variety of mirror structures including sampled grating reflectors, coupled ring resonators, etc. Performance characteristics that are critical in wavelength division multiplexed applications and high capacity links include single frequency (wavelength) mode operation where the quality of single frequency is defined by parameters like the side mode suppression ratio (SMSR) and in cases where coherent transmission is employed, by the line width of the laser output. In widely tunable lasers, the laser must be tuned over 10s of nanometers range, for example 30-40 nanometers to cover transmission bands used in engineering and operating wavelength multiplexed fiber links, for example the C-band, and in other cases over 40 to 100 nm and over 100 nm tuning is desirable. In prior state of the art, tunable lasers are generally designed so that the maximum power can be extracted from one of the laser mirrors, the primary output mirror, which is connected to the additional elements and the optical fiber. For example in a transmitter, these additional elements include an optical amplifier, an optical modulator, optical waveguides, monitor photodiodes, wavelength locking optics, and the optical fiber. In a receiver these elements coupled to the primary output of the tunable laser may include an optical amplifier, an optical mixer, photodiodes and an optical fiber. In prior art, the requirement to extract maximum optical power from one of the mirrors in order to maximize power into a modulator, optical fiber or other element, leads to tradeoffs in the mirror designs and overall laser tuning design and laser characteristics. For example, maximum power out from a primary mirror requires a decrease in peak mirror reflectivity for the primary output mirror. This results in laser performance tradeoffs including a decrease in the side mode suppression ratio (SMSR) and decrease in the wavelength tuning selectivity and wavelength stability and possibly laser line width. The flatness (or slow roll off) of the periodic wavelength reflection peaks is another important parameter that is critical to achieving wide range tenability. Optimizing these parameters, as well as other parameters, is critical to meeting optical link and network performance requirements while providing low cost, low power, small size and high reliability tunable lasers and transmitters.
For communications applications, it is desirable to make a higher-level building block called the tunable optical transmitter by integrating the tunable laser with an optical data modulator. Different types of optical data modulators are used depending on the performance and application and generally fall into the non-coherent and coherent categories. Integration of these two components, potentially with other components, results in decrease in cost, size, yield per wafer, transmitter power dissipation, and increased transmitted output power as well as other desired optical transmission properties. Monolithic integration of the widely tunable laser with an optical data modulator can be accomplished on the same common semiconductor substrate like indium phosphide or silicon. Other types of laser-modulator integration utilize hybrid integration techniques where the laser, optical amplifier, and data modulator are placed onto a common waveguide communication substrate or interposer made of a suitable material like glass, silicon or silica nitride.
It is desirable when constructing coherent optical communications systems to also integrate the tunable laser into the optical receiver and if possible integrate the coherent optical transmitter and receiver together.
The choice of both the tunable laser design and optical data modulator design are important in the manufacturing and characterization process of tunable transmitters and receivers, determining how the resulting wavelength tunable transmitter will perform in a fiber optic link or network and how well the laser and modulator can be integrated to reduce cost, power dissipation, size without sacrificing reliability and also maintaining required system performance. Additionally the choice of tunable laser design is important in integration of a coherent optical receiver and integration of an tunable optical transmitter and receiver together.
The more successful monolithically integrated widely tunable lasers utilize a linear arrangement of front and rear mirrors, gain section, phase tuning section, and power measurement electrodes. Other designs arrange the mirrors in Y-branches with a common output waveguide and similar mirror, gain section and phase section-tuning elements. The Y-branch configurations have utilized sampled grating mirrors like the linear designs or ring resonators as the mirrors in the Y-section of the laser and a broadband non-tunable reflector at the primary optical emission output located on the single arm portion of the Y. Both designs may contain additional optical filter elements that may also be tunable.
For data modulators, the semiconductor Mach-Zehnder Modulator (MZM) is a preferred modulator design due to the ability to integrate it with the tunable laser, optical data modulation characteristics, low electrical drive voltage requirements, compact size and programmable transmission characteristics. In prior art, integrated transmitters are fabricated by coupling light from the primary laser output mirror (the higher output power mirror via a single waveguide to the modulator input). In the case of a MZM, the modulator input is split into two optical waveguide paths (called arms) and then combined into a common data modulated output waveguide and a secondary waveguide that can be used for optical monitoring. Data is modulated onto the tunable laser output by driving one or both of the MZM paths (arms) with an electronic data signal that affects the physical properties of the MZM waveguides via electrical electrodes or interconnects. This prior state of the art approach, where the input to the MZM requires the power to be split leads to performance tradeoffs, design tradeoffs, and increased susceptibility to fabrication and environmental variations. Examples of transmitter characteristics that are required and lead to tradeoffs include operation over the wide laser wavelength tuning range and required range of temperature and environmental conditions, laser chirp, modulator drive voltage and optical data extinction ratio, output power and signal to noise ratio.
There are multiple problems with prior tunable lasers that utilize a single primary mirror output and integrated laser-MZM transmitters. For the laser these problems include for example optical frequency output quality, difficulty in measuring mirror and laser output characteristics in a manufacturing environment, expensive and complex laser output characterization and programming methods, and increased cost and complexity in reliability and testing. When the laser and modulator are integrated, problems associated with prior tunable lasers that utilize a single primary output mirror connected to the MZM directly or through other optical elements include non-balanced power splitting that occurs in elements like power splitters due to fabrication tolerances, temperature or other environmental or fabrication variations.
Therefore, there is a need for a tunable laser that simplifies characterization, testing and calibration in a manufacturable manner and that works in combination with an integrated optical data modulator that is able to achieve optimal operating and transmission performance with minimal design, operating, and reliability tradeoffs.